\pagedir TRT\bodydir TRT\pardir TRT\textdir TRT
\definefontfeature[otfnaskhi]
   [mode=node,language=dflt,script=arab,
    init=yes,medi=yes,fina=yes,calt=yes,
    rlig=yes,liga=yes,dlig=yes,curs=yes,
    mark=yes,mkmk=yes]
\starttext
\font\test=SimpleNaskhi*otfnaskhi at 48pt\test

تِّ : تِّ
\stoptext